1. Field of the Invention
The invention pertains to a transition between a microstrip circuit and a waveguide. More particularly, the transition which is the subject of the invention corresponds to a transition for a transmit circuit of an outside transmit/receive unit. The invention pertains also to the outside transmit/receive unit.
2. Related Art
Bidirectional-satellite transmissions are being called on to develop, within the mass marker sector, low-cost solutions are currently being sought so as to be able to disseminate them on a large scale. For a bidirectional system, it is preferable to use one antenna rather than two antennas.
A known problem with respect to the transmission standards defined by the public organizations for allocating frequency is that they require that transmitted signals should come within a specific transmission spectrum template. Another known problem relates to the coupling between transmission and reception. Specifically, the same antenna being used for transmission and for reception, the high-power transmitted signals will disturb the low-power received signals. Although the transmit and receive bands are seperated, it is necessary to have good filtering on reception in order to reduce the saturation of the low noise amplifier.
The local oscillator used for transmission may be at a frequency lying very near the transmission band and precludes the possibility of an effective bandpass filter for so close a frequency. Furthermore, the signal corresponding to the local oscillator is as amplified as the transmitted signal. It is known to use an additional bandstop filter to attenuate the frequency line corresponding to the local oscillator.
FIG. 1 represents an exemplary outside unit I according to the state of the art. At the output of the mixer 3, a bandpass filter 4 selects the transmission band and attenuates the signal corresponding to the frequency of the local oscillator 2. However, such filteing is not sufficient and requires the addition of a bandsrop filter 5 to attenuate the signal corresponding to the frequency of the local oscillator 2 by at least 50 dB. A power amplifier 6 then amplifies the signal to be transmitted before the signal is transformed into an electromagnetic wave by a transition 7 between a microstrip technology circuit and a waveguide 8 linked to a horn 9. The use of the bandstop filter 5 has the effect of eliminating the component corresponding to the local oscillator 2. Thus, the frequency of the local oscillator 2 is no longer a nuisance in respect of the transmission. Moreover, the possible echo of the signal corresponding to the frequency of the local oscillator 2 being greatly attenuated, therefore minimizing local oscillator signal contribution to the saturation of the low noise amplifier of the reception circuit.
On the other hand, the embodying of a microstrip technology filter requires a lengthening of the microstrip lines and the addition of amplifiers 11 and 12. Microstrip technology does not permit a good quality factor to be obtained in respect of the embodying of the bandstop filter 5. It is relatively difficult to have 50 dB of attenuation, this requiring the constraints on the bandpass filter 4 to be increased.